Sensor Technology
Magnet design for giant magnetoresistance multi-turn position sensors
By Stephen Bradshaw, product applications engineer, Christian Nau, product applications manager, and Enda Nicholl, strategic marketing manager, all with Analog Devices
T
rue power-on multi-turn sensors based on giant magnetoresistance (GMR) sensing technology are set to revolutionise the position sensing market in both industrial and automotive use cases due to reduced system complexity and maintenance requirements compared to existing solutions. This article describes some of the key factors that must be considered when designing a magnetic system to ensure reliable operation even in the most demanding applications. It also introduces a magnetic reference design that is available for early adoption of the technology.
Introduction
The multi-turn sensor is essentially a magnetic write and electronic read memory combined with a conventional magnetic angle sensor to provide a highly accurate absolute position. The magnetic write process requires the incident magnetic field to be maintained with a specific operating window. Magnetic write errors may occur if the magnetic field is either too high or too low. It is essential to design the system magnet carefully and to consider any stray magnetic fields that might interfere with the sensor as well as mechanical tolerances over the life of the product. Small stray magnetic fields could cause an error in the measured angle while larger stray magnetic fields could cause a magnetic write error leading to a gross turn count error.
Magnetic reference design goals A careful understanding of the system requirements is necessary to design the optimum magnet and shielding. Generally, the looser the system requirements, the larger and more expensive the magnet solution required to achieve the target specifications. Analog Devices is developing a series of magnetic reference designs addressing various
18 October 2024 Figure 1. A thermal coefficient comparison of the operating window vs. a typical SmCo magnet.
mechanical, stray field, and temperature requirements that can be adopted by customers of the ADMT4000 true power-on multi-turn sensor. The first design developed by ADI covers systems with relatively loose tolerances: sensor to magnet placement of 2.45mm ± 1mm, a total displacement of the sensor to the axis of rotation of ±0.6mm, operating temperature range of –40˚C to +150˚C, and stray magnetic field shield attenuation of greater than 90 per cent.
Magnetic considerations When designing the magnet, there are some key considerations to take into account and the following section provides a high-level view of the main aspects to consider when designing for the GMR sensor.
Magnet material
The GMR sensor operates in a defined magnetic window (16mT to 31mT)1 addition, the maximum and minimum
Components in Electronics ; in
operating range has a thermal coefficient (TC) as can be seen by the red traces in Figure 1. Selecting a magnet material with a TC that matches that of the GMR sensor will maximise the allowed variation of the operating magnetic field. This allows for greater variation in the strength of the magnet and/or the placement tolerance of the magnet with respect to the sensor. Low-cost magnetic materials such as ferrites have a much higher TC than the GMR sensor, which would limit the operating temperature range compared with materials such as samarium- cobalt (SmCo) or neodymium-iron-boron (NeFeB).
Understanding the TC of the chosen magnetic material as well as the variation in the magnetic field strength due to manufacturing variations allows the required magnetic field strength at room temperature (25°C) to be determined. Design simulations may then be carried out at room temperature with a high degree of confidence that the system will operate as expected over the full temperature range. In Figure 1, the solid green traces represent a window of the magnetic field strength that the magnet should be designed to produce over the active area of the GMR sensor. This window is reduced from the maximum and minimum operating window of the GMR sensor due to variations in the manufacturing process of the magnetic material. The green dotted lines show the maximum and minimum expected magnetic field due to a typical manufacturing
The operating window is subject to change pending the release of the ADMT4000.
variation of >5%. 1
Magnet simulation
The simulation of the magnet within the mechanical operating environment can take different forms. There are two types of simulation commonly used to design the magnet: an analytic simulation or finite element analysis (FEA). The analytic simulation solves the
magnetic field using the bulk parameters of the
magnet being simulated
Figure 2. The reference design magnet.
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